1. Field of the Invention
The present invention relates generally to the pipeline charging of preheated coal into coke ovens and the problems incurred by the presence of coal fines in such a system and the recovery of those coal fines.
2. Description of the Prior Art
In the known methods for controlling and recovering coal fines in a preheated pipeline coal charging system, the preheated and pressurized coal is directed firstly through a primary cyclone into which the larger particles of coal are deposited. The balance of the fluid, composed of varying sizes of coal fines and not gases, next passes through a secondary cyclone system where the bulk of the fine solids entrained in the hot gases are deposited. The hot gases, carrying less than 0.5% of the original coal charged, are then piped through a scrubber, where the balance of the entrained coal fines are removed and reclaimed.
About 90% of the coal particles are deposited in the primary cyclone with most of the balance of 10% being deposited in the secondary cyclone system. The coal deposited in the primary cyclone is almost entirely larger than 100 mesh size. The finer coal, deposited in the secondary cyclone system, is of a fine mesh ranging in size from about 100 mesh (150 microns) down to smaller than 400 mesh, with the bulk of these fines, more than 90%, being less than 325 mesh (44 microns) in size.
The coal from both the primary cyclone and the secondary cyclone system is fed into a conveyor system which, in turn, feeds the coal to a distribution bin. The coal from the distribution bin, still at an elevated temperature, is then pumped into coke ovens via pipelines. Thus the pipeline charging of the coke ovens is complete. Most of the high pneumatic pressure or steam pressure used to pump the coal through is bled off from the pipeline prior to the coke oven and is channeled to the by-product train; the balance passes through the coke oven and then into the by-product train.
Problems have developed in this system of pipeline charging. The finer particles of coal, recovered from the secondary cyclone system, develop into "carry-over" when introduced into the coke ovens as the oven is being charged. As this fine coal enters a hot coke oven, a portion of it rapidly devolatilizes into gases, causing, consequently, a high velocity in the gases that are discharged into the gas collecting system. This high exit velocity of rapidly devolitilized gases carries a larger quantity of entrained fine coal particles than is found with conventional coke oven charging systems. This increased quantity of coal fines has been found to overload the charging liquor systems of pipeline charged coke ovens. Also, the fine coal particles that are blown out of the oven are accompanied by fine tar droplets that result from the coal devolitilization. The mixture tends to compact in such places as the standpipes and the charging mains. This accumulation significantly reduces the efficiency of the coke oven by-products system and is difficult and costly to remove.
The result of these problems is a relegation of the coal fines collected in the secondary cyclone system to the status of low grade boiler fuel in an effort to eliminate fine particles of less than 100 mesh from the system. Due to the increasing scarcity of metallurgical grade coal and the commensurate increasing cost thereof, the use of that coal for low grade boiler fuel is quite uneconomical. A method for utilizing these coal fines in coking operations in a manner which eliminates the problems described previously is needed.